Motion, Acceleration, Energy of Motion

Chapter 2 Motion, Acceleration, Energy of Motion

Chapter 2.1 Motion

2.1 Motion • Objectives • Compare frames of reference. • Distinguish between speed and velocity. • Calculate when a moving object will arrive at a given point. • Make a graph to solve a distance-time problem.

2.1 Motion • Imagine you are traveling across the ocean on a ship. You see another ship getting closer. • Are both ships moving or is only the ship you are on moving?

2.1 Motion • It isn't always easy to decide when objects are moving. • You judge motion in relation to stationary objects. This is called a frame of reference.

Frame of Reference • A frame of reference is a place or object that you assume is fixed. • You observe how objects move in relation to that frame of reference. What does this photographs tell you about the motion of the car?

You use the earth’s surface as your frame of reference most of the time. However, you experience moving frames of reference when you ride in a vehicle, such as a car, a bus, or an elevator. Two persons with the same frame of reference see the motion of an object the same way. Both boys on the bus see the ball drop straight down. Two persons with different frames of reference see an object’s motion differently. Both boys on the bus see the ball drop straight down. To a bystander, the ball moves in two directions—forward with the bus & down toward its floor.

MOTION – Reference Frames • http://www.youtube.com/watch?v=7FYBG5GSklU

Measuring Motion Speed = Distance/Time • Speed is the distance an object travels in a certain amount of time. • To calculate speed, you divide the distance traveled by the time it took to travel that distance. If the cyclist rode his bike 8 km to school in 20 minutes, you can calculate his speed in km/h as follows: Speed : 8 km/20 min x 60 min/1 h = 24 km/h

Velocity • The speed and the direction of an object's motion are called velocity. • Since a moving object always travels in some direction, velocity is a more accurate term for describing motion. • A weather vane indicates wind direction. If it blows at 40 km/h what is its velocity?

Constant Speed • A moving object that doesn't change its speed travels at constant speed. • Constant speed means equal distances are covered in an equal amount or time. • A distance-time graph of constant speed forms a straight line.

Average Speed • A distance-time graph of a marathon shows that the runner's speed changed several times. To find the speed of the runner during the entire race, you need to calculate his average speed. Average speed is equal to the total distance traveled divided by the total time. What was the average speed of the marathon runner?

Practice Problems pg.36 • 1. A kayak races 100 m in 50 s. What is its speed? • Speed = distance / time • 100 m/ 50 s = 2 m/s • Now do 3 & 5 3. Distance=3m/s x 35s = 105 m Distance = 3m/s x 60s/min x 60 min = 10,800 m 5. 7.5 km / 5h = 1.5 km/h

Relativity and Space -Time • We know observations of motion depend on your frame of reference. • Yet, experiments show the speed of light is always the same, regardless of the motion of the light source or the motion of the observer! • To understand this, think about a rocket docked on the earth and another rocket traveling directly toward the sun at great speed… • The light from the sun reaches both rockets at the same speed (300 million meters/s).

Relativity and Space -Time • The constant speed of light is the basis for Albert Einstein's special theory of relativity. • He reasoned that space and time are connected into one whole, called space-time. • You are constantly traveling through this combination of space and time. • When you stand still, you travel only through time.

Relativity and Space -Time • When you move, you travel through space and time. • If you could move at close to the speed of light, you would travel quickly through space and slowly through time. http://www.youtube.com/watch?v=KYWM2oZgi4E&feature=related

Check and Explain pg. 38 • Answer questions 1 & 2. Use complete sentences. • 1. A frame of reference is usually fixed and is used to describe the motion of objects relative to it. • 2. the velocity of the bird include the direction the bird is moving.

Chapter 2.2 Acceleration

2.2 Acceleration • Objectives • Define operationally the acceleration of an object. • Relate motion in a circle to acceleration. • Contrast acceleration and constant speed. • Make a graph showing acceleration.

Acceleration • Any change in velocity is called an acceleration. • Acceleration can be speeding up, slowing down, or changing the direction of the motion. • All of these accelerations, or velocity changes, require an outside force.

Acceleration • Imagine you are competing as a speed skater at the Winter Olympics. • During practice, you learned that three changes in speed were the key to winning the race: • speed up whenever possible • slow down with control when necessary • and change direction as smoothly as possible. • When you mastered these three things (changes in velocity), you are ready to compete.

Change in Velocity • The rate at which velocity changes occur is called acceleration. • To calculate acceleration you divide the change in velocity by the amount of time. Acceleration = Change in velocity (final velocity – starting velocity) time

Suppose you ride a bike on a straight path to school at a velocity of 4 m/s. As you get closer, you hear the school bell. In 3 s, you speed up to 10 m/s. How would you calculate your acceleration? Change in Velocity = final velocity - starting velocity 10 m/ s – 4 m/ s = 6 m /s Acceleration =

Positive Acceleration A Graph of Positive Acceleration To think about positive acceleration, imagine a car waiting at a stoplight. When the light turns from red to green, the driver steps on the accelerator, and the car speeds up. As the car moves faster, you feel the change in motion as your body is pushed back against the seat. How many seconds did the car travelat a constant positive acceleration?

Negative Acceleration A Graph of Negative Acceleration To think about negative acceleration, imagine a car slowing down. The car's velocity decreases over a certain amount of time. This type of velocity change is also called deceleration. During what 3 second interval did the car decelerate fastest?

Change in Direction • Also, whenever the direction of a moving object changes, the velocity of the object changes. • Remember, any change in velocity is acceleration—even if the speed of the object remains the same. Each change in direction represents a change in velocity.

Skills Builder page 41 A Racer's Acceleration Each change in direction represents a change in velocity. Suppose you are a race car driver. At the beginning of the race, all of the cars start from rest and accelerate straight down the race track before reaching the first corner. Your car's speed after the first few seconds is given in the table. Use the data in the table to plot a speed-versus-time line.

Skills Builder • Answers: • No. Constant acceleration would produce a linear graph. • Final velocity, starting velocity and time. • 8 m/s2 , 8 m/s2, 6 m/s2, and4 m/s2 respectively. • Greatest: 0-2 sec., Least: 3-4 sec. • 26 m/s – 0 m/s = 6.5 m/s2 4s • The friend will be in the lead.

Motion in a Circle • An object moving in a circle or a curve is constantly changing direction, therefore, the object is accelerating. • Acceleration caused by motion in a circle is called centripetal acceleration. What would happen to the eraser if the girl let go of the string? Why?

2.3 Energy of Motion

2.3 Energy of Motion • Objectives • Define energy. • Explain the law of conservation of energy. • Compare and contrast potential energy and kinetic energy. • Infer the gravitational potential energy of everyday objects.

Energy of Motion • Changes in motion occur constantly in the world around you. • Cars move on busy highways. • Tons of rock hurtle down mountainsides. • Underground water is brought to the surface for crop irrigation. • What gives the cars, rocks, and water the ability to move?

Energy and Change • Any change in motion requires energy. • Energy is the ability to do work. • When work is done, a change occurs. • One might say that energy is the source of change. The archer uses energy to pull the bow's string toward her to change the string's position. When she lets go, energy in the string transfers to the arrow, and the arrow moves forward.

Potential Energy • Stored energy has the ability to do work and is called potential energy. • Potential energy is associated with position.

Kinetic Energy • Energy of motion is called kinetic energy. • The amount of kinetic energy depends on the moving object's mass and velocity. • To calculate kinetic energy, you multiply one half of the object's mass times the square of its velocity.

Kinetic Energy • If the mass of a bowling ball is 4 kg and its velocity is 5 m/s, how much kinetic energy does it have?

Conservation of Energy • Energy cannot be created or destroyed. • This is known as the law of conservation of energy. • Energy can change into other forms, but the total amount of energy never changes.

Gravity and Energy • You may infer that a rock on top of a cliff has potential energy because of its position. • Once over the cliff’s edge, the force of gravity pulls the rock downward. The rock's energy is due to both its position on the cliff and the force of gravity. • This type of energy is called gravitational potential energy.

Science & Technology • Turn to page 49 in your text book. • Read “SCIENCE & TECHNOLOGY” Great Potential • Look at figure 2.19 • Answer the question: • How is a water wheel like a dam?

Motion, Acceleration, Energy of Motion